1. Field of the Invention
The present invention relates to an apparatus and method for preventing electrical shock due to contact with an uninsulated electronic component, such as a passive infrared detector. More particularly, the present invention relates to a mechanical apparatus, such as a finger guard, which restricts physical contact with an uninsulated component, such as infrared detector, carrying a voltage, and an electrical apparatus, such as a current limiting circuit, which limits current supplied to the uninsulated, to prevent electrical shock due to inadvertent or intentional contact with the uninsulated component.
2. Description of the Related Art
A motion sensor switch, such as Model AT277W motion sensor switch manufactured by Hubbell, Inc., includes a passive infrared detector (PIR), and an ambient light detector. Motion sensor switch can be used, for example, as an occupancy detector which shuts off lights after a delay in a room when no one is present in the room, and turns on the lights in the room when a person enters the room. Motion sensor switch also can be used, for example, as a motion sensor for an alarm system.
The motion sensor senses motion in an occupied area, such as an office, conference room in a building, or a home, and in turn controls the lighting loads to save energy. The motion sensor detects a change in the infrared energy radiating from the occupant as the occupant move in and out of or between the PIR lenses sensing lobes. The PIR detector has a pass band in the 8-14 .mu.m infrared range.
If a person enters the monitored area, the person changes the amount of infrared energy being detected by the PIR detector. Therefore, the magnitude of the signal output by PIR detector, which is representative of the amount of detected infrared energy, will change. The detection circuit in the motion detector device processes this signal, and outputs a signal indicating that the amount of infrared energy received by the infrared detector has changed.
The control circuit interprets the signal output by the IR detection circuit, along with the signal provided by ambient light detector. If the signal provided by ambient light detector indicates that the ambient light in the monitored area is low (e.g., very little natural light is present in the monitored area), the control circuit will turn on or increase the brightness of the lights in the monitored area However, if the signal provided by ambient light detector indicates that the ambient light in the monitored area is sufficient (e.g., due to sunlight, etc.), the control circuit may not turn on or brighten the lights, or may only brighten the lights slightly. In either event, control of the lights is based on the signals provided by the infrared detector and ambient light detector.
If a person (occupant) then leaves the monitored area for a period of time exceeding the "off delay", the circuit of the motion sensor turns off the lighting load in the monitored area.
The infrared detector and ambient light detector employed in a motion detector switch of the type described above are non-insulated electronic components which during operation typically carry voltages on at least a portion of their outer casing, or on exposed uninsulated terminals. Because these voltages can create an electrical shock hazard, UL Standards covering uninsulated live parts (i.e., UL773A, effective Aug. 30, 1998) require either that physical access to the infrared detector and ambient light detector be restricted, or that the current provided to infrared detector and ambient light detector be limited to less than 500 .mu.A.
Accordingly, in a motion sensor switch of the type described above, a fresnel lens, which is made of, for example, polyethylene, flexible plastic or any other suitable material, is installed in front of infrared detector and ambient light detector. The lens is most transmissive in the 8-14 .mu.m infrared range, but will also allow visible ambient light (400-700 nm wavelength) to pass. This enables the infrared detector and ambient light detector to receive infrared energy and ambient light, respectively, while also preventing physical contact with the infrared detector and ambient light detector.
Although a lens of the type described above is typically suitable for preventing contact with the infrared detector in the motion detector switch while in place, tampering (e.g., cutting, prying with a tool) with the motion detector switch may result in the lens being removed. Once the lens has been removed, the surfaces of the infrared detector and ambient light detector are exposed and can be contacted by, for example, a person's finger, which could result in the person receiving an electrical shock from the voltage and current being carried by those detectors. Furthermore, the lens is made of a material which will usually melt when exposed to fire. Therefore, due to the required infrared transmissive properties, the lens is not capable of passing a 5-inch flame test required in UL773A.
Although it is possible to provide a flame resistant lens over the infrared energy and ambient light detectors, these type of lenses can be very expensive, and thus not provide a cost effective solution.
Accordingly, a continuing need exists for an apparatus and method which provides added safety in preventing electrical shock due to contact with infrared and ambient light detectors in, for example, a motion detector switch, which does not significantly obstruct the passage of infrared energy and ambient light to the infrared and ambient light detectors, respectively, and which also fully complies with UL773A.